The invention relates to use of a cellulose derivative having a group containing an aromatic group as a separating agent for a chemical substance. The invention method applies to separation of optical isomers, geometrical isomers and polymers having different molecular weight ranges from each other. They have not easily been separated in the state of prior arts.
Generally, the physiological activity of a racemic compound often differs from that of an optically active compound. For example, in the field of medicines, pesticides or the like, it is sometimes necessary to resolve optical isomers for the purposes of preventing adverse reactions and improving the medicinal effects per unit dose. A mixture of optical isomers has been divided by a preferential crystallization process or diastereomer process. However, variaties of the compound which can be optically resolved by these processes are limited and most of these processes require a long time. Under these circumstances, development of a convenient chromatographic resolution process has eagerly been demanded.
The chromatographic resolution of optical isomers has been investigated from old times. For instance, cellulose and a triacetate thereof have been successfully used as column-chromatographic resolving agents in optical resolution. The cellulose and cellulose triacetate are those belonging to cellulose I and cellulose triacetate I, respectively. However, substances which can be resolved by said cellulose or a derivative thereof are limited and the resolving ability of them is insufficient.
After intensive investigations, the inventors have found surprisingly that a cellulose derivative having a group containing an aromatic ring has excellent ability in separation of chemical substances and isomers, in particular optical isomers. The invention has been completed on the basis of the finding.
The invention relates to a method for separating a chemical substance from a mixture containing the same, which comprises the step of treating said mixture with a cellulose derivative having a group containing an aromatic ring, a separating agent comprising the cellulose derivative; particles of the separating agent; a packing material of the particles; and a chromatographic column filled with the agent.
Though the reasons why the cellulose derivatives having an aromatic ring used in the present invention have excellent effects for resolving the optical isomers have not been elucidated yet, it may be considered that the ordered asymmetic structure of cellulose and the aromaticity and rigidity of the aromatic group exert a great influence on the resolution of the optical isomers.
The cellulose derivative according to the invention is preferred to have a number-average degree of polymerization of 5 to 5000, preferably 10 to 1000, and particularly 10 to 500. The average degree of substitution of the cellulose derivative having an aromatic ring is defined by the following formula:                               Average          ⁢                      xe2x80x83                    ⁢          degree                                              of          ⁢                      xe2x80x83                    ⁢          substitution                      =            Number      ⁢              xe2x80x83            ⁢      of      ⁢              xe2x80x83            ⁢      substituents      ⁢              xe2x80x83            ⁢      in      ⁢              xe2x80x83            ⁢      the      ⁢              xe2x80x83            ⁢      molecule              Number      ⁢              -            ⁢      average      ⁢              xe2x80x83            ⁢              d        ⁢        egree            ⁢              xe2x80x83            ⁢      of      ⁢              xe2x80x83            ⁢      polymerization      
The average degree of substitution of the cellulose derivatives having an aromatic ring of the present invention is 1 to 3.4, preferably 1.8 to 3.2.
The unreacted hydroxyl groups in the aromatic cellulose derivative containing aromatic rings may further be esterified, carbamoylated or etherified so far as its capacity of resolving optical isomers is not damaged.
The cellulose derivative of the invention may include those in which part or all of the hydrogen atoms of the hydroxyl groups have been replaced with an aromatic group or a group containing an aromatic group. A substituent may be attached to cellulose by way of an intermediate linkage such as an ester, an ether and an urethane. The term xe2x80x9caromatic groupxe2x80x9d includes that derived from an aromatic ring having 6 to 20 carbon atoms, an aralkyl group having 6 to 20 carbon atoms in the aryl portion and 1 to 4 carbon atoms in the alkyl portion and a heteroaromatic ring having 3 to 20 carbon atoms. The ring may further have a substituent thereon, such as an alkyl group, nitro group, a halogen, an amino group, an alkyl-substituted amino group, cyano group, hydroxyl group and carboxyl group.
Now, the description will be made on processes for the production of the substances of the present invention. The cellulose derivatives substituted through an ester group include, cellulose benzoate for example. The esterification reaction to obtain them may be carried out by a known process, (See xe2x80x9cDai-Yuki Kagakuxe2x80x9d 19, xe2x80x9cTennen Kobunshi Kagakuxe2x80x9d I published by Asakura Book Store, p. 124). Examples of the esterifying agents include benzoyl derivatives having the following structures such as benzoyl chloride: 
The reaction solvent may be any solvent such as pyridine and quinoline, so far as it does not inhibit the esterification reaction. Frequently a catalyst such as 4-(N,N-dimethylamino)pyridine is effective in accelerating the reaction. Other aromatic derivatives may be obtained by the esterification reaction in the same manner as described above.
The cellulose derivative substituted through an ether group may be obtained by a known process for etherifying cellulose. Generally, they are obtained by reacting cellulose with an aromatic derivative having a leaving group in the presence of a base. This process has been disclosed in, for example, N. M. Bikales, L. Segel, xe2x80x9cCellulose and Cellulose Derivativesxe2x80x9d p. 807 and xe2x80x9cDai-Yuki Kagakuxe2x80x9d 19 published by Asakura Book Store, p. 93. Processes for producing cellulose ethers having an aromatic ring of a high degree of substitution includes that of Husemann et al. (xe2x80x9cMakromol. Chem,xe2x80x9d, 176, 3269 (1975)) and that of Nakano et al. (xe2x80x9cThe Processings of ISWPCxe2x80x9d #1983, Vol. 1, 33).
The cellulose derivatives substituted through an urethane group may be produced by a conventional process wherein an isocyanate is reacted with an alcohol to form a urethane.
For example, these compounds may be produced by reacting an isocyanate having an aromatic ring with cellulose in the presence of a Lewis base catalyst such as a tertiary amine base or a Lewis acid catalyst such as a tin compound.
The disubstituted urethanes may be synthesized in the same manner as in the above-mentioned esterification reaction using a disubstituted carbamoyl halide or the like.
In using the resolving agent of the present invention containing the cellulose derivatives having an aromatic ring as the principal component for the purpose of resolution, it is preferred to employ a chromatographic method. The preferred chromatographic methods include liquid, thin layer and gas chromatography.
In using the separating agent of the present invention in the liquid or gas chromatography, there may be employed a method wherein the aromatic ring-containing cellulose derivative is packed into a column directly or in the form supported on a carrier or a method wherein a capillary column is coated with said cellulose derivative.
Since the chromatographic separating agent is preferably in the form of granules, the aromatic ring-containing cellulose derivative to be used as the resolving agent is preferably ground or shaped into beads. The particle size which varies depending on the size of a column or plate used is generally 1 xcexcm to 10 mm, preferably 1 to 300 xcexcm. The particles are preferably porous.
It is preferred to support the aromatic ring-containing cellulose derivative on a carrier so as to improve the resistance thereof to pressure, to prevent swelling or shrinkage thereof due to solvent exchange or to reduce the number of theoretical plates. The suitable size of the carrier which varies depending on the size of the column or plate used is generally 1 xcexcm to 10 mm, preferably 1 to 300 xcexcm. The carrier is preferably porous and has an average pore diameter of 10 xc3x85 to 100 xcexcm, preferably 50 to 50,000 xc3x85. The amount of said cellulose derivative to be supported is 1 to 100 wt. %, preferably 5 to 50 wt. %, based on the carrier. The carrier is preferred to have a ratio of the pore size to the particle size in the range of not larger than 0.1.
The aromatic ring-containing cellulose derivative may be supported on the carrier by either chemical or physical means. For example, the cellulose derivative is dissolved in a suitable solvent, then the solution is mixed uniformly with a carrier and the solvent is distilled off under a reduced pressure or by heating. Alternatively, the cellulose derivative is dissolved in a suitable solvent, the resulting solution is mixed homogeneously with a carrier and the mixture is dispersed in a liquid incompatible with said solvent by stirring to diffuse the solvent. The cellulose derivative thus supported on the carrier may be crystallized, if necessary, by heat treatment or the like. Further, the state of the supported cellulose derivative and accordingly its resolving power can be modified by adding a small amount of a solvent thereto to temporarily swell or dissolve it and then distilling the solvent off.
Both porous organic and inorganic carriers may be used, though the latter is preferred. The suitable porous organic carriers are those comprising a high molecular substance such as polystyrene, polyacrylamide or polyacrylate. The suitable porous inorganic carriers are synthetic or natural products such as silica, alumina, magnesia, titanium oxide, glass, silicate or kaolin. They may be treated on the surface so as to improve the affinity with the separating agent of the invention. The surface-treatment may be conducted with use of an organosilane compound or by plasma polymerization.
In using the cellulose derivatives in the optical resolution, the resolving characteristics thereof may vary sometimes depending on the physical states thereof such as molecular weight, crystallinity and orientation, even though they are chemically similar. Therefore, the cellulose derivatives may be subjected to a physical or chemical treatment such as heat treatment or etching in the course of or after shaping them suitable for use.
As to the developers for the liquid chromatography, solvents in which the aromatic ring-containing cellulose derivative is soluble cannot be used. However, the developers are not particularly limited when the aromatic ring-containing cellulose derivative is chemically bound to the carrier or when it is cross-linked.
In the thin layer chromatography, a layer having a thickness of 0.1 to 100 mm and comprising the resolving agent in the form of particles of about 0.1 xcexcm to 0.1 mm and a small amount of a binder is formed on a supporting plate.
The aromatic ring-containing cellulose derivative may be spun into a hollow fiber in which an eluent containing the compound to be resolved is to flow so that the resolution is effected by virtue of the adsorption of the compound on the inner wall of the filament. In another embodiment, the cellulose derivative is spun into an ordinary filament, which is then bundled in parallel and placed in a column so as to take advantage of the adsorption on the surface thereof. In the membrane resolution process, the resolving agent may be used in the form of a hollow fiber or film.
The resolving agent of the present invention comprising the aromatic ring-containing cellulose derivative as a principal constituent is effective for the resolution of compounds. Particularly, it is quite effective for the resolution of optical isomers which are quite difficult to resolve in the prior art. This way the optical isomer mixture to be treated in the invention includes a compound having the asymmetric center and one having the molecular asymmetry such that either one of the optical isomers may be preferably adsorbed on the separating agent of the invention.
We explain the term, xe2x80x9cheteroaromatic derivativexe2x80x9d of the invention. It preferably includes an ester between a cellulose and a carboxylic or carbamic acid having a heteroaromatic group or a substituted heteroaromatic group. The heteroaromatic group has 3 to 20 carbon atoms. Among them, an acyl group, a carbamoyl group and alkyl group are more preferable. Especially an acyl and carbamoyl are best. Embodiments are illustrated below. 
n being an integer of 0 to 5, preferably 0 or 1.
The term xe2x80x9csubstituted aromatic esterxe2x80x9d herein involves a carboxylic acid ester having an aromatic group wherein one or more hydrogen atom(s) is (are) replaced with one or more atom(s) or atomic group(s). The alcoholic moiety of the ester comprises the above-mentioned polysaccharide. The carboxylic acids have preferably acyl groups of the following formula: 
wherein X, Y and Z each represents an alkyl group, alkenyl group, alkynyl group, nitro group, halogen atom, amino group, alkyl-substituted amino group, cyano group, hydroxyl group, alkoxy group, acyl group, thiol group, sulfonyl group, carboxyl group or alkoxycarbonyl group, l and m represent the numbers of X, Y and Z groups (l being an integer of 1 to 5, m being an integer of 1 to 7 and n being an integer of 0 to 5, preferably 0).
30 to 100%, preferably 85 to 100%, on average of the hydroxyl groups of the polysaccharide moiety in the aromatic ester should be esterified with the carboxylic acid.
The balance of the hydroxyl groups may be present in the form of free hydroxyl groups or they may be esterified, etherified or carbamoylated so far as the resolving capacity of the resolving agent is not damaged.
Now, the description will be made on the process for the esterification to form a cellulose derivative to be used in the present invention. A substituted benzoic ester may be produced by a known process (See, for example, xe2x80x9cDai-Yuki Kagakuxe2x80x9d 19, xe2x80x98Tennen Kobunshi Kagaku Ixe2x80x99 published by Asakura Book Store, p. 124). Examples of the esterifying agents include benzoyl derivatives of the following formula, particularly, benzoyl chloride: 
Any solvent may be used in the reaction so far as it does not inhibit the esterification. Pyridine and quinoline are preferred. Frequently, a catalyst such as 4-(N,N-dimethylamino)pyridine is effective in accelerating the reaction.
The esters may also be obtained by reacting the corresponding carboxylic anhydride or a combination of the carboxylic acid with a suitable dehydrating agent with the polysaccharide.
Other ester derivatives of the present invention may be synthesized according to the above-mentioned process for the synthesis of the substituted benzoic esters.
The resolving agent of the present invention containing the substituted aromatic ester as the effective component is effective for the resolution of various compounds. Particularly, it is quite effective for the resolution of optical isomers which are quite difficult to resolve. Either one of the optical isomers to be resolved is selectively adsorbed on the resolving agent.
Particularly, according to the present invention, the resolving and adsorbing properties of a resolving agent are varied by introducing a substituent therein to improve the intended resolving effects, particularly, optical resolving effects. For example, the effects of cellulose tribenzoate will be compared with those of a chlorine-substitured derivative thereof. A Trxc3x6ger""s base which can not be optically resolved by the tribenzoate can be resolved by tris-4-chlorobenzoate with an xcex1-value of 1.25. Benzoin which is resolved by the tribenzoate with an xcex1-value of 1.14 exhibits an xcex1-value of 1.26 in the resolution by the tris-3-chlorobenzoate. Similarly, benzoin, 2-phenylcyclohexanone and mandelamide are optically resolved with the tris-3,5-dichlorobenzoate far more easily than with the tribenzoate. Though the relationship between the remarkable change in resolving characteristics and the substitution has not fully been elucidated, the change might be caused by a complicated combination of factors such as influence of a substituent on the shape of the molecule, physico-chemical properties, such as polarizability, hydrogen bonding ability and polarity, of the substituent and electronic effects of the substituent on the xcfx80-electron system of the aromatic ring.
It is thus apparent from the present invention that the substituent has a quite effective influence on the modification of the resolving characteristics of the resolving agent and the development of resolving agents having various characteristics has been made possible. These effects are expected also in various other resolving agents have polysaccharides other than cellulose as the skeleton.
Particularly, by modifying the polysaccharides with the substituent having a sufficient length, the asymmetrical structure of the polysaccharide is further developed to obtain a higher capacity of resolving the optical isomers.
The above-mentioned objects of the present invention can be attained with the resolving agent containing an aralkylcarboxylic ester of a polysaccharide as the effective component.
The resolving agent of the present invention exhibits preferably different absorbing capacities on respective optical isomers of a compound.
The term xe2x80x9caralkylcarboxylic acidsxe2x80x9d herein involves substitution derivatives of acetic acid and they include aliphatic carboxylic acids having an aromatic substituent in the molecule and various substitution derivatives of them, preferably arylacetic acid and aryloxyacetic acid derivatives. Acrylic acid derivatives, benzoic acid derivatives and propiolic acid derivatives must be excluded even if they contain an aromatic ring. The aromatic rings include, for example, phenyl, naphthyl, phenanthryl and anthryl rings. They may be bonded with the skeleton in any manner.
Examples of these compounds include the following compounds: 
The aromatic ring may have various substituents so far as the effecs of the present invention are not damaged.
30 to 100%, preferably 85 to 100%, on average of the hydroxyl groups of the polysaccharides forming the polysaccharide/aralkylcarboxylic ester of the present invention should be esterified with the carboxylic acid. The balance of the hydroxyl groups may be present in the form of free hydroxyl groups of they may be esterified, etherified or carbamoylated so far as the resolving capacity of the resolving agent is not damaged.
The esterification for forming the compounds used in the present invention may be conducted by a known process for the esterification of cellulose or amylose (see, for example, xe2x80x9cDai-Yuki Kagakuxe2x80x9d 19, xe2x80x98Tennen Kobunshi Kagaku Ixe2x80x99 published by Asakura Book Store, p. 124, reference 1). Common esterifying agents are anhydrides and halides of the corresponding carboxylic acids, particularly acid chlorides.
It is preferred to use a tertiary amine base or a Lewis acid as a catalyst. Any reaction solvent may be used so far as it does not inhibit the reaction. For example, pyridine or quinoline which acts also as the base is used frequently. Further, a catalyst such as 4-(N,N-dimethylamino)pyridine is effective in accelerating the invention.
The corresponding carboxylic acid in combination with a suitable dehydrating agent may be reacted with the polysaccharide.
Since most of the polysaccharides used as the starting material have a low reactivity, it is preferred that they are activated by dissolution/reprecipitation or dissolution/freeze-drying treatment or by using a reaction solvent in which the polysaccharides are soluble.